Spotify has Jay-Z as well.[0] I would say it's hard to really quantify which library has the larger selection as most of the differences would be completely obscure music that no one listens to. However, I haven't seen any artist on GPM that isn't on Spotify while the opposite can't be said.

Spotify has the ability to upload music as well, however that is contingent on you having premium.[1]

[0] https://open.spotify.com/artist/3nFkdlSjzX9mRTtwJOzDYB?si=j1...

[1]https://community.spotify.com/t5/Desktop-Windows/Can-you-upl...